Recently Sn plated steel sheet, Pb-Sn plated steel sheet, Cu plated steel sheet or Zn plated steel sheet have been used as a plated steel sheets suitable for soldering. However, these plated steel sheets easily corrode during packing or after assembling unless they are post-treated. Therefore, post-treatments such as chromate treatment, phosphate treatment or coating of anti-corrosion primer are applied on said steel sheets.
Chromate treatment is not satisfactory because more than 0.1 mg/dm.sup.2 of Cr.sup.6+ ion is necessary in order to provide sufficient corrosion resistance. However, if more than 0.05 mg/dm.sup.2 of Cr.sup.6+ is used, a strong flux is necessary for soldering. The use of strong flux causes deterioration of the working environment and decrease of corrosion resistance of the soldered surface. Therefore, the amount of Cr.sup.6+ has to be limited to less than 0.05 mg/dm.sup.2, which does not provide sufficient corrosion resistance.
On the other hand, the application of an anti-corrosion primer causes decrease of solderability similar to chromate treatment and also becomes easily soiled by handling at bending or soldering. Moreover, corrosion resistance of soiled areas after soldering is less satisfactory than in other areas.
In order to provide a plated steel sheet for soldering, it is important to apply on the plated steel sheet after plating a suitable post-treatment which does not detract from solderability and which increases the corrosion resistance.
However, use of the post-treatment methods of the prior art cannot provide a plated steel sheet excellent in corrosion resistance and solderability because solderability and corrosion resistance show contrary tendency to each other. This tendency becomes apparent after aging, because the oxide film on the plated steel sheet increases with passage of time and decreases the solderability.
In view of the facts described above, it is necessary to improve the post-treatment after plating. There is some prior art relating to a process for production of plated steel sheet for soldering, for example, Japanese Patent Application Laid-Open No. Sho 63-88085, Japanese Patent Application Laid-Open No. Sho 63-115670, and Japanese Patent Application Laid-Open No. Sho 63-224769. These prior art processes have exhibited superior properties by coating post-treatment substances onto the plated steel sheet. However, stain resistance and corrosion resistance at stained parts have been insufficient.
We have examined the prior art post-treatment processes in detail to solve the problems mentioned above and developed the present invention.
By coating the present post-treatment solution onto the plated steel sheets, not only solderability and corrosion resistance but also resistance to finger printing and corrosion resistance of the finger-printed areas have increased.